1. Field of the Invention
The present invention relates to a polarization insensitive semiconductor optical amplifier or amplifying apparatus and an optical communication system or network using this optical amplifier.
2. Related Background Art
Generally, a semiconductor optical amplifier or an amplifying apparatus comprises a semiconductor laser structure including an active layer and a cladding layer, and amplifies an input light by means of a bias current, below a threshold, injected into the laser structure. In the optical communication field, the optical amplifier has been developed as a device for compensating for an optical loss that occurs in optical fibers or at connections between optical fibers.
However, there has been the problem of a polarization dependency of the optical amplification factor (i.e. the optical amplification factor differs depending on the different polarization modes of an input light) when a semiconductor optical amplifier is used in optical fiber communication systems. Generally, the state of polarization of an output light, which is transmitted through the optical fiber, is unstable, so the level of an output light from the optical amplifier will not be stable when such light from the optical fiber is input into such an amplifier having the above-discussed polarization dependency. Further, the fluctuation of such output burdens a light receiving system regarding its dynamic range and the like. This is a vital drawback which limits the scale of the communication system.
Therefore, various conventional methods of fabricating a polarization insensitive optical amplifier have been performed. Among them, a method of using a strain quantum well structure is proposed as a method for solving the polarization dependency of optical gain in the active layer. The strain quantum well structure is used for both controlling the oscillation wavelength and reducing an oscillation threshold gain in the field of semiconductor lasers, and thus this structure is a remarkably useful technique.
Generally, in order to utilize a strain quantum well structure as a polarization insensitive optical amplifier, the gain for TM mode light is equalized with or made larger than that for TE light. More in particular, the degeneracy in a valence band is solved by the effect of strain, and hence band structures of heavy and light holes are respectively shifted. Thus, the energy gap between the ground level of electrons in the conduction band and the level of heavy holes in the valence band is approximately made equal with or made slightly larger than the energy gap between the ground level of electrons in the conduction band and the level of light holes in the valence band. When there is no polarization dependency of gain other than that of the optical gain, those energy gaps are equalized with each other. When there exists a gain dependency on polarization that is other than that of the optical gain, the latter energy gap, concerning the light holes, is made smaller. In general, optical confinement for TE light is larger than that for TM light, so the latter energy gap, concerning the light holes, is made smaller when considering such difference in optical confinement.
Several methods have been proposed for creating the strain necessary for obtaining the above-discussed desired energy levels.
First, a method for imparting a biaxial tensile stress to a well structure is proposed as disclosed in Japanese Patent Laid-Open Application No. 1-251685 (1989). On a reference first semiconductor layer (i.e. a substrate or a cladding layer), a second semiconductor layer, having a lattice constant smaller than that of the first semiconductor layer, is formed. Hence a biaxial tensile stress is imparted to the second semiconductor layer. The energy level of the light holes in the valence band is shifted in a direction for narrowing its band gap, by imparting the biaxial tensile stress to the well layer. As a result, the energy level of light holes in the valence band approaches the energy level of heavy holes in the valence band, and hence a desired energy level is obtained.
Second, a method for imparting a biaxial tensile stress to a barrier layer is proposed as disclosed in Japanese Patent Laid-Open Application No. 4-27183 (1992). Similar to the first method, an energy level of light holes in a valence band of the barrier layer is shifted. As a result, a well for the light holes is shallowed, leading to a shift in the energy level, and a desired level results.
Third, a method of fabricating an active layer having a strained well layer (a biaxial tensile stress exists) and a non-strained well layer is proposed as described in Japanese Patent Laid-Open Application No. 1-257386 (1989).
However, those prior art methods have respectively their own advantages and disadvantages. The first prior art method has an advantage in that a relatively large amount of energy shift can be obtained by a slight strain amount and thus a desired effect can be achieved by a relatively small amount of strain. On the other hand, an energy shift of the conduction band occurs simultaneously with an energy shift of the valence band due to the effect of strain. Conversely, the second prior art method has a drawback in that a relatively large amount of strain is needed, compared to the first prior art method, to attain a desired effect, although the wavelength, at which a gain is obtained, hardly changes.
The third prior art method has an advantage in that freedom in design is increased by the combination of two well layers and a desired effect can be readily obtained. However, the amount of strain needs to be increased, compared to the first prior art method.
A semiconductor optical amplifier is used for amplifying a signal light generated by driving a semiconductor laser, and this amplifier can be replaced by a structure resembling the semiconductor laser. If a wavelength range of gain of the amplifier largely changes due to the effects of strain, its construction material has to be unfavorably changed.
Further, dislocation in a strained lattice can be prevented by reducing its layer thickness to a value less than a critical thickness, but its life time under a long-term driven condition decreases as the amount of strain increases. Moreover, the growth condition for fabricating the strained lattice becomes severe as the amount of strain increases. Therefore, it is not advantageous to increase the amount of strain excessively.